Description will be made of the background art by way of a shovel as an example.
A shovel shown in FIG. 13 includes a lower propelling body 1 of crawler type, a upper slewing body 2 mounted on the lower propelling body 1 so as to be rotatable about an axis O vertical to the ground, and a working attachment A provided on the upper slewing body 2 so as to be movable upwardly and downwardly. The working attachment A includes a boom 3, an arm 4, a bucket 5, a boom cylinder 6, an arm cylinder 7, and a bucket cylinder 8.
Known examples of such shovels include a hybrid-type shovel (see Patent Document 1) and an electrically driven-type shovel (hereinafter will be referred to as an “electrically-driven slewing-type shovel”). Such an electrically-driven slewing-type shovel uses a slewing power source comprising either an electric motor or a hydraulic motor and an electric motor. In the electrically-driven slewing-type shovel, a slewing driving force is generated by driving an electric slewing motor by an electric storage device. During slewing braking, on the other hand, regenerative electric power and regenerative braking force are generated by the electric slewing motor while regenerating slewing energy by charging the electric storage device with the regenerative electric power in the electrically-driven slewing-type shovel.
In the electrically-driven slewing-type shovel, the regenerative electric power cannot sufficiently be taken in the electric storage device when the electric storage device has a high charge level. Therefore, the slewing energy cannot sufficiently be regenerated.
The same holds true for cases where the electric storage device has a decreased capacity due to temperature, aging deterioration, or the like.
In such a situation, an energy loss incurs for the reason that a portion of the slewing energy which has failed to be regenerated has to be consumed by a regenerative resistance and a like reason. Further, the slewing braking force becomes insufficient, which gives rise to a state so-called “overrun of slewing” such that a desired stop position is passed through.
When the electric storage device is overcharged, it is possible that the performance and life of the electric storage device are deleteriously affected.    Patent Document 1: WO 2006/004080